The present invention is generally related to radio frequency (RF) signal receivers, and more particularly to an improved method and apparatus for eliminating interference due to spurious signals generated in synthesized RF signal receivers.
Conventional high frequency, synthesized, single sideband (SSB) receivers must be capable of receiving RF signals having frequencies at any possible multiple of 100 Hz between 2 mHz and 30 mHz. Since conventional SSB receivers may include one or more stages of signal multiplication, spurious signals, commonly referred to as "whistler spurs", can be generated which produce an audible tone at the output of the SSB receiver, making the particular RF signal frequency substantially unuseable.
The same phenomenon can occur in dual conversion, synthesized, frequency modulated (FM) receivers. In the case of such FM receivers, spurious signals, commonly referred to as "self quieters", cause the output of the FM receiver to be quieted (i.e. squelch opens) even though an RF signal is not being received. These self quieters also may render the particular RF signal frequency substantially unusable.
In the past, RF signal frequencies producing such whistler spurs or self quieters could not be used either because of the interference created or because of degradation of receiver performance requirements. This problem may be alleviated somewhat by selecting between two or more different intermediate frequency (IF) stages when the receiver is being manufactured. Assuming that the operating RF signal frequencies have been assigned to a particular reciever, the receiver sometimes can be manufactured with IF stage circuitry operating at a particular IF frequency that does not produce a whistler spur or self quieter on any of the assigned RF signal frequencies. For example, one of three different IF stages each having a different IF frequency can be selected to eliminate a whistler spur or self quieter. However, since the frequency of the IF stage is fixed during the manufacturing of the receiver, changes in the operating RF signal frequencies after manufacture can again produce a whistler spur or self quieter. Since it is very likely that the operating RF signal frequencies will be changed after manufacture of a receiver, selection between several different frequency IF stages during manufacture does not adequately solve the problem of whistler spurs and self quieters.